The present invention pertains to the art of magnetic resonance imaging. It finds particular application in conjunction with medical diagnostic imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention may have broader applications.
Heretofore, magnetic resonance images were each composed of a plurality of views, e.g. 256 views. The data for each view was collected subsequent to the application of a different amplitude phase encode gradient along a phase encode axis. More specifically, each of the 256 views was collected in conjunction with one of 256 phase encode gradient amplitudes. The phase encode gradient was stepped in equal steps or intervals along the phase encode axis to assume each of 256 amplitudes. Analogously, for a coarser 128 view image, the phase encode gradient was stepped in 128 equal steps and, for a higher resolution 512 view image, the phase encode gradient was stepped in 512 equal steps.
In magnetic resonance imaging, the field of view, FOV, varied with the number of phase encode gradient steps or views, N, and the resolution .DELTA., specifically: EQU FOV=N.DELTA. (1).
Thus, for a selected field of view, a finer resolution can be achieved by increasing the number of views. The phase encode gradient steps or intervals, .DELTA.G were fixed for a given field of view and varied inversely therewith, i.e.: EQU .DELTA.G=(2.gamma.FOV .multidot.t.sub..phi.).sup.-1 ( 2),
where .gamma. is the gyromagnetic ratio and t.sub..phi. is the duration of the gradient. Thus, to decrease the field of view the phase encode gradient steps were increased accordingly. The maximum phase encode gradient, G.sub.max was inversely proportional to the resolution, specifically: EQU G.sub.max =(4.sub..gamma. t.sub..phi. .DELTA.).sup.-1 ( 3)
Thus, for a selected resolution the maximum phase encode gradient was fixed regardless of the selected field of view. For a given field of view, the size of the phase encode gradient steps was fixed. To improve resolution, the number of steps was increased which raised the maximum phase encode gradient.
Commonly, magnetic resonance imagers were preprogrammed such that the operator could select one of a limited number of field of view dimensions and one of a limited number of resolutions.
One of the disadvantages of the prior art scanners was that the imaged patient or subject rarely filled the field of view. Rather, a significant portion of the field of view was dead space or air of no diagnostic value. Collecting views corresponding to the dead space not only wasted scanning time but also caused artifacts which degraded the overall image quality.
In accordance with the present invention, a method and apparatus are provided for eliminating views corresponding to dead space to increase imaging speed and to decrease artifacts.